Process for pre-expanding thermoplastic resin particles

Abstract

In a process for pre-expanding thermoplastic resin particles which comprises releasing into a low pressure zone a mixture of water and thermoplastic resin particles containing a volatile blowing agent, the mixture being maintained at an elevated temperature under pressure in a pressure vessel, the improvement which comprises releasing the mixture through a plurality of orifice plates in series in the direction of release of the mixture. According to the process of the present invention, the average cell diameter of the obtained pre-expanded particles is larger than that of the pre-expanded particles obtained by means of conventional methods. Therefore, the molded articles prepared from the pre-expanded particles according to the present invention are small in shrinkage and excellent in surface appearance, and are utilizable for use as shock absorbing, wrapping materials or structural materials such as impact-absorbing materials for cars (for example, a car bumper), which require excellent dimensional precision and heat insulating building materials.

What we claim is: 
     
       1. In a process for preparing pre-expanding thermoplastic resin particles which comprises releasing into a low pressure zone a mixture of water and thermoplastic resin particles containing a volatile blowing agent, said mixture being maintained at an elevated temperature under pressure in a pressure vessel, the improvement which comprises releasing said mixture through a plurality of orifice plates in series in the direction of release of the mixture. 
     
     
       2. The process of claim 1, wherein said thermoplastic resin is a polyolefin resin. 
     
     
       3. The process of claim 2, wherein said polyolefin resin is polypropylene resin. 
     
     
       4. The process of claim 2, wherein said polyolefin resin is polyethylene resin. 
     
     
       5. The process of claim 1, wherein said orifice plates are at least one member selected from the group consisting of an orifice type plate, a nozzle type plate and a Venturi type plate. 
     
     
       6. The process of claim 1, wherein said orifice plates have plural holes.